Method and apparatus for simultaneously sync hunting signals

ABSTRACT

A method and apparatus for simultaneously sync hunting signals is described. In one embodiment of the invention, a computer implemented method comprises initializing a first and second subset of a set of per-alignment state machines, receiving a first and second signal, and simultaneously sync hunting the first signal with the first subset of the set of per-alignment state machines and the second signal with the second subset of the set of per-alignment state machines.

NOTICE OF RELATED APPLICATION

This is a continuation of U.S. Provisional Application No. 60/280,694,entitled “A Method and Apparatus for Processing Multiple CommunicationsSignals in One Clock Domain”, filed Mar. 31, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to communication networks. More specifically, theinvention relates to processing bit streams.

2. Description of the Related Art

A digital transmission line that uses wire-pair and coaxial cable isknown as a T-carrier. T-carriers include T1 and T3 lines. A T1 line is apoint to point digital communications circuit that carries 24 64 kbits/schannels (“Digital Hierarchy-Formats Specification”, American NationalStandards for Telecommunications, ANSI T1.107, 1995). The bits on the T1circuit are sent as frames. Each frame consists of 24 8 bit channelsresulting in 192 bits per frame (“Digital Hierarchy-FormatsSpecification”, American National Standards for Telecommunications, ANSIT1.107, 1995). The frames are sent at a rate of 8,000 frames per second(“Digital Hierarchy-Formats Specification”, American National Standardsfor Telecommunications, ANSI T1.107, 1995). This transfer rate providesan aggregate payload data rate of approximately 1.544 Mbits/s (“DigitalHierarchy-Formats Specification”, American National Standards forTelecommunications, ANSI T1.107, 1995). A framing bit forsynchronization increases the size of each frame to 193 bits. Theframing bit cycles through a framing bit pattern. A receiver searchesfor this framing bit pattern to achieve synchronization of the bitstream it is receiving. This bit format is referred to as digital signallevel 1 (DS1).

A T-3 line is a digital transmission circuit that supports 28 T1 lines.The bit rate for a T1 line is approximately 44.736 Mbits/s. The bitformat of the bit streams carried over T3 lines is referred to asdigital signal level 3 (DS3). DS1 signals are multiplexed into DS3signals. The multiplexing process is a 2 step process (“The Fundamentalsof DS3”, 1992). Four DS1 signals are bit by bit interleaved to form aDS2 signal. Seven DS2 signals are multiplexed to form a DS3 signal.

FIG. 1 (Prior Art) is a diagram of a DSn deframer. A DS3 bit stream 101and a clock signal 103 enter a line interface unit 105. The lineinterface unit 105 feeds the bit stream 101, clock signal 103, and avalid bit stream 107 into a DS3 deframer 102. The DS3 deframer 102 synchunts the bit stream received from the line interface unit 105. Each ofthe seven DS2 subchannels (a signal bit stream 113 and subchannel bitstream 115) carried in the DS3 signal 101 is fed into individual DS2deframers 106. Individual clocks are generated for each DS2 deframerwith the DS2 clock rate. From each of the DS2 deframers 106, a bitstream 117 and a subchannel bit stream 119 is fed into four DS1deframers 110, for a total of twenty-eight DS1 deframers 110. A clockfor each of these DS1 deframers is generated with a DS1 clock rate.Hence, a total of 36 clocks (1 DS3 clock+7 DS2 clocks+28 DS1 clocks) aregenerated to deframe a single DS3 bit stream. A deframed bit stream 121is sent to a destination external to the DSn deframer from each of theDS1 deframers 110.

Deframing more than one DS3 bit stream requires a network element with a1:1 relationship of DSn deframers to DS3 bit streams. Alternatively, aDSn deframer with a 1:n relationship to DS3 bit streams becomesincreasingly complicated and costly since the number of deframers andclocks increase linearly with the number of DS3 bit streams to beprocessed.

SUMMARY OF THE INVENTION

A method and apparatus for simultaneously sync hunting signals isdescribed. According to one embodiment of the invention, a method isprovided for initializing a first and second subset of a set ofper-alignment state machines and receiving a first and second signal,and simultaneously sync hunting the first signal with the first subsetof the set of per-alignment state machines and the second signal withthe second subset of the set of per-alignment state machines.

These and other aspects of the invention will be better described withreference to the Detailed Description and the accompanying Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by referring to the followingdescription and accompanying drawings that are used to illustrateembodiments of the invention. In the drawings:

FIG. 1 (Prior Art) is a diagram of a DSn deframer.

FIG. 2 is a diagram of units of a network element according to oneembodiment of the invention.

FIG. 3 is an exemplary diagram of deframing slices of the DSn deframingblock 250 of FIG. 2 according to one embodiment of the invention.

FIG. 4A is a diagram illustrating data flow through the deframing slice301 of FIG. 3 according to one embodiment of the invention.

FIG. 4B is a diagram illustrating data flow through either of thedeframing slices 303 or 305 of FIG. 3 according to one embodiment of theinvention.

FIG. 5 is a diagram of the DS3 deframer 320 of FIG. 3 according to oneembodiment of the invention.

FIG. 6 is a diagram of the DS2 deframer 322 of FIG. 3 according to oneembodiment of the invention.

FIG. 7 is a diagram of the DS1 deframer 324 according to one embodimentof the invention.

FIG. 8A is a flow chart for DS3 sync hunting according to one embodimentof the invention.

FIG. 8B is a flow chart for performing block 821 of FIG. 8A according toone embodiment of the invention.

FIG. 9 illustrates an example of storing DS3 bits in per-alignment statemachines as potential framing bits according to one embodiment of theinvention.

FIG. 10 is a diagram illustrating organization of the per-alignmentstate machines in the sync hunt per-alignment memory 513 of FIG. 5according to one embodiment of the invention.

FIG. 11A is a flow chart for performing DS2 synchronization huntingaccording to one embodiment of the invention.

FIG. 11B is a flow chart for performing block 1117 of FIG. 11A accordingto one embodiment of the invention.

FIG. 12 illustrates an example of storing bits in DS2 per-alignmentstate machines as potential alignment bits according to one embodimentof the invention.

FIG. 13 is a diagram illustrating organization of the per-alignmentstate machines in the sync hunt per-alignment memory 621 of FIG. 6according to one embodiment of the invention.

FIG. 14A is the flow chart for initializing the per-alignment statemachines for DS1 super frame sync hunting according to one embodiment ofthe invention.

FIG. 14B is a flow chart for performing block 1415 of FIG. 14A accordingto one embodiment of the invention.

FIG. 15 is an exemplary illustration of FIG. 14A according to oneembodiment of the invention.

FIG. 16A is a flow chart for DS1 extended super frame sync huntingaccording to one embodiment of the invention.

FIG. 16B is a flow chart for performing block 1621 of FIG. 16A accordingone embodiment of the invention.

FIG. 17 is an exemplary illustration for storing F-bits in per-alignmentstate machines for sync hunting DS1 extended superframe according to oneembodiment of the invention.

FIG. 18 is a diagram illustrating the organization of per-alignmentstate machines in the memory unit 321 of FIG. 3 according to oneembodiment of the invention.

FIG. 19 is a flowchart for DS3 deframing performed by the DS3 deframinglogic 525 of FIG. 5 according to one embodiment of the invention.

FIG. 20 is a flowchart for DS2 deframing performed by the DS2 deframinglogic 625 of FIG. 6 according to one embodiment of the invention.

FIG. 21 is a flowchart for DS1 deframing performed by the DS1 deframinglogic 725 of FIG. 7 according to one embodiment of the invention.

FIG. 22 is a flowchart for change of frame alignment feed forwardingaccording to one embodiment of the invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toprovide a thorough understanding of the invention. However, it isunderstood that the invention may be practiced without these specificdetails. In other instances, well-known protocols, structures andtechniques have not been shown in detail in order not to obscure theinvention. Although the invention has been described with respect toDS3, DS2 and DS1 signals, the invention can also be applied to othersignaling formats including E3, E2, E1, J1, etc.

FIG. 2 is a diagram of units of a network element according to oneembodiment of the invention. In FIG. 2, a receiving unit 201 receivesmultiple DS3 signals. The DS3 signals can loop to a transmit bufferingunit 242. The receiving unit 201 is also connected to an opticaltransmitting unit 225 and a receive buffering unit 202. The receivebuffering unit 202 is connected to a DSn deframing block 250. Theoptical transmitting unit 225 processes the DS3 signals from thereceiving unit 201 for optical transmission (e.g., mapping the DS3signals to STS formatting).

The DSn deframing block 250 includes a DS3 deframing unit 203, a DS2deframing unit 205, and a DS1 deframing unit 209. Some signals that flowinto the DSn deframing block 250 enter the DS3 deframing unit 203. TheDS3 deframing unit 203 is connected to the DS2 deframing unit 205. TheDS2 deframing unit 205 connects to the DS1 deframing unit 209.

DS1 formatted bit streams are received at the DS1 receive and transferunit 217 of the network element. In one embodiment of the invention, theDS1 bit streams are received from T1 lines (not shown) that areconnected to the DS1 receive and transmit unit 217. DS1 signals can becarried in a number of ways including as SONET payload, microwave, etc.The DS1 receive and transmit unit 217 connects to a receiving DS1 buffer207 and a transmitting DS1 buffer 237. The receiving DS1 buffer 207 iscoupled to the DS1 deframing unit 209. DS1 signals received at the DS1receive and transmit unit 217 follow a path to the DS1 deframing unit209 via the receiving DS1 buffer 207.

The DS1 deframing unit 209 is coupled to an external memory unit 211.The DS1 deframing unit 209 is also coupled to a DS2 framing unit 239 anda DS3/DS1 data buffer 213. The DS3/DS1 data buffer 213 is coupled to aprotocol receiving unit 215.

A protocol transmitting unit 231 is connected to a DS3/DS1 data buffer233. The protocol transmitting unit 231 performs various functions suchas protocol encapsulation. The data buffer 233 is connected to a DS3framing unit 241 and a DS1 framing unit 235. The DS1 framing unit 235connects to the transmitting DS1 buffer 237. Bit streams framed by theDS1 framing unit 235 follow a path to the DS1 receive and transmit unit217 via the transmitting DS1 buffer 237. The DS1 framing unit 235 alsoconnects to the DS2 framing unit 239. The DS2 framing unit 239 connectsto the DS3 framing unit 241. The DS3 framing unit 241 is coupled to thetransmit buffering unit 242 and the optical transmitting unit 225.

An optical receiving unit 229 connects to the transmit buffering unit242. The optical receiving unit 229 performs various functions such asdemapping STS formatted signals into DS3 signals. The optical receivingunit 229 also connects to the receive buffering unit 202.

FIG. 3 is an exemplary diagram of deframing slices of the DSn deframingblock 250 of FIG. 2 according to one embodiment of the invention. InFIG. 3, multiple deframing slices 301, 303 and 305 are shown. In thisexample, each deframing slice has two DS3 inputs 302, 304 from a set ofone or more receiving units and two inputs 316, 318 from a set of one ormore optical receiving units. Each input flows into one of a set ofbuffers 306-309. For each of the deframing slices 301, 303, 305, theinput 316 flows into the buffer 307 and then goes into a selecting unit311; the DS3 input 302 flows into a buffer 306 and then into theselecting unit 311; the input 318 flows into the buffer 308 and theninto a selecting unit 312; the DS3 input 304 flows into the buffer 309and continues into the selecting unit 312; and the input selected by theselecting units 311 and 312 then flow into a multiplexer 313.

In one embodiment of the invention, the set of buffers 306-309 areasynchronous First In First Out buffers (FIFOs). The inputs are writteninto the buffers 306-309 at the DS3 rate and read at the rate of thedomain clock. In one embodiment of the invention, the domain clock runsat 100 Mhz in order to process 2 DS3 bit streams (each DS3 running atapproximately 45 Mhz) per deframing slice. However, embodiments of thepresent invention are not so limited, as the domain clock can run atother clock rates that run faster than the sum of the clock rates of theincoming signals. Since each DS3 bit stream may originate from sourcesrunning at slightly different clock rates, valid bits accompany DS3 dataread out of the asynchronous FIFOs in the clock domain.

For each of the deframing slices 301, 303, and 305, the multiplexer 313multiplexes the input selected by the selecting units 311 and 312 beforesending the multiplexed input into a DS3 deframer 320. Each deframerslice includes the DS3 deframer 320, a DS2 deframer 322, and a DS1deframer 324.

Each individual deframer processes successively lower bandwidthchannels. Since each deframer handles two DS3 bit streams worth of data,though, each deframer actually processes approximately the same totalnumber of bits. The DS3 deframer 320 handles two DS3 channels. The DS2deframer 322 processes fourteen DS2 channels. The DS1 deframer 324processes fifty-six DS1 channels. Input flows from the DS3 deframer 320to the DS2 deframer 322, and then to the DS1 deframer 324. From the DS1deframer 324 of each of the deframing slices 301, 303, and 305, bitsflow into the DS1 data buffer 213 of FIG. 2. The bits from each of thedeframing slices 301, 303 and 305 are respectively stored in one of thecorresponding buffers 325-327 for bit to byte conversion. Once the datais converted, it is multiplexed by the multiplexing unit 328 andtransmitted to a protocol receive unit.

In addition to the DS3 inputs 302, 304 and inputs 306, 308, thedeframing slice 301 receives DS1 bit streams from a receiving T1 buffer348. The receiving T1 buffer includes a set of buffers 335 to bufferindividual DS1 signals. The buffered DS1 signals are multiplexed by amultiplexing unit 333 of the receiving T1 buffer 348. The multiplexer333 passes the multiplexed DS1 signals to the deframing slice 301. Whenthe deframing slice 301 receives DS1 bit streams, it multiplexes the DS1bit streams with one of the deframed bit streams 302, 304, 316 or 318 ofthe deframing slice. These inputs are multiplexed at a multiplexer 315before being sent to the DS1 deframer 324. The DS1 deframer 324 of eachof the deframing slices 301, 303 and 305, is connected to a memorycontroller 321. The memory controller 321 handles read and writeoperations to an external memory unit 323. The external memory unit 323stores states for sync hunting which is described later in relation toFIGS. 7, 8A-8B, and 11A-11B. The memory controller 321 serves the DS1deframer 324 of each deframing slice 301, 303, 305 at the same time. Inan example of six deframer slices, each receiving two DS3 bit streams,the memory controller 321 iterates through 168 (6 slices*28 DS1 channelsper DS3) channels of possible DS1 sync hunting. In another embodiment ofthe invention, the order of iteration is subchannel 0-27 for the firstDS3 input bit stream (channel 0) followed by subchannels 0-27 for thesecond DS3 input bit stream (channel 1). In one embodiment of theinvention, the memory controller 321 serves all read requests beforeserving all write requests in the order previously described. Processingrequests in this fashion holds read to write bus turnaround to a minimumof once per 168 bus cycles in one embodiment of the invention.

In another embodiment of the invention, every deframing slice 301, 303,305 only receives one DS3 bit stream input. In another embodiment of theinvention, each deframer slice receives one DS3 bit stream input and aset of DS1 bit streams. In another embodiment of the invention, eachdeframing slice receives inputs from two sets of DS1 bit streams. Inanother embodiment of the invention, a deframing slice can have Ninputs, each of the N inputs independently configurable for either a DS3input or a set of DS1 inputs.

In one embodiment, each deframer 320, 322, 324 processes its set ofchannels in a time division multiplex fashion. For example, the DS3deframer 320 works on the pair of DS3 channels in alternating cycles.The DS2 deframer 322 works on 14 DS2 channels in a circulatory fashion.The order the DS2 deframer 322 circulates through the DS2 subchannelsdepends on the order in which they are deframed by the DS3 deframer 320.In other words, the DS3 deframer 320 pushes DS2 subchannels into the DS2deframer 322. Likewise, the order the DS1 deframer 324 circulatesthrough its 56 DS1 subchannels is dictated by the DS2 deframer 322.

FIG. 4A is a diagram illustrating data flow through a deframing sliceaccording to one embodiment of the invention. In FIG. 4A, a data bitstream 401, a data bit stream 402, and a channel select signal 403 flowinto a multiplexer 443 of a deframing slice 400. The data bit streams401 and 402 may include bits from the original DS3 signals and validbits. From the multiplexer 443, a multiplexed data bit stream 405, avalid bit stream 407 and a channel bit stream 409 flow into a DS3deframer 450. From the DS3 deframer 450, a data bit stream 406 and avalid bit stream 408 flow into a DS2 deframer at 452. A subchannel bitstream 410 flows into the DS2 deframer 452 and a context memory 411. Thecontext memory 411 includes a per-channel state memory and a sync huntper-alignment memory for each pair of subchannels, which will bedescribed herein. Information 404 from the context memory 411 flows intothe DS2 deframer 452. Updates 444 are written back to the context memory411. A data bit stream 412 and a validity bit stream 414 flow from theDS2 deframer 452 into a DS1 deframer 454. The subchannel bit stream 416flows from the DS2 deframer 452 to both the DS1 deframer 454 and acontext memory 419. Information 430 from the context memory 419 flowsinto the DS1 deframer 454. Updates 432 are written back to the contextmemory 419. A data bit stream 418, a valid bit stream 420, and asubchannel bit stream 422 flow from the DS1 deframer 454 out of thedeframing slice 400.

To accommodate 2 DS3 signals (transmitted at approximately 44.736 Mhzeach) feeding into a deframing slice, the deframers run at approximately100 Mhz. Each of the DS3 bit streams appears to flow through 50 Mhzdeframers. Having the deframers outrun the bit streams insures that thedeframers will be fast enough to deframe all incoming bits. In addition,although each bit stream needs a set of state for deframing(specifically, sync hunting which is a necessary aspect of deframing),the faster rate enables 2 DS3 bit streams to be deframed with one core.A single core logic for 2 DS3 bit streams provides a savings of space.In another embodiment of the invention, a faster clock speed for thedeframers, such as 200 Mhz, enables a single core logic to process 4 DS3bit streams. In another embodiment of the invention, a deframing slicereceives N channels or inputs processed at M bits at a time. In such anembodiment, the core clock exceeds the following: sum(n=1 . . . N,clockrate[n]/M).

FIG. 4B is a diagram illustrating data flow through a deframing slicereceiving D53 and DS1 input according to one embodiment of theinvention. In FIG. 4B, a data bit stream 401 and a channel select signal403 flow into the multiplexer 443. From the multiplexer 443, amultiplexed data bit stream 405, a validity bit stream 407, and achannel bit stream 409 flow into the DS3 deframer 450. Although anotherdata bit stream does not flow into the multiplexer 443, the multiplexer443 multiplexes the data bit stream 401 with a stream of stuffing bitsfor half of the domain clock's cycles to create the multiplexed bitstream 405.

The DS3 deframer 450 processes the streams 405, 407 and 409 andgenerates a data bit stream 406, a validity bit stream 408, and asubchannel bit stream 410 which flow into the DS2 deframer 452. Thesubchannel bit stream 410 also flows into a context memory 411. Thecontext memory 411 includes a per-channel state memory and a sync huntper-alignment memory for each pair of subchannels. The per-channel statememory and the sync hunt per-alignment memory for each deframer will bedescribed later herein with references to FIGS. 6-12. Information 404from the context memory 411 flows into the DS2 deframer 452.

The DS2 deframer 452 processes the streams 406, 408, 410 and theinformation 404 from the context memory 411 to generate a data bitstream 413, a validity bit stream 415, and a subchannel bit stream 417.The streams 413, 415, and 417 flow into a multiplexer 480. Updates 444are written back to the context memory 411 from the DS2 deframer 452.Data bit streams also flow into the multiplexer 480 from a receiving T1buffer 482. A data bit stream 427, a validity bit stream 425, and asubchannel bit stream 424 flow into the multiplexer 480 from thereceiving T1 buffer 482. The data bit stream 427 and the data bit stream413 are multiplexed to generate a data bit stream 412. The validity bitstreams 415 and 425 are multiplexed to generate a validity bit stream414. The subchannel bit streams 417 and 424 are multiplexed to generatethe bit stream 416. The streams 412, 414, 416 flow into the DS1 deframer454. The subchannel bit stream 416 also flows into a context memory 419.Information 430 from the context memory flows into the DS1 deframer 454.The context memory 419 and the information 430 stored in the contextmemory 419 are described later.

The DS1 deframer 454 processes the bit streams 412, 414, 416 and theinformation 430 from the context memory 419. After processing, the DS1deframer 454 generates a data bit stream 418, a validity bit stream 420,and a subchannel bit stream 422. Updates 432 are written back to thecontext memory 419 from the DS1 deframer 324.

FIG. 5 is a diagram of a DS3 deframer according to one embodiment of theinvention. In FIG. 5, a DS3 deframer 500 receives bit streams from asource external to the DS3 deframer 500. The two DS3 data bit streams401 and 402 of FIG. 4A feed into a multiplexing unit 443. The channelselect signal 403 also feeds into the multiplexing unit 443. Themultiplexing unit 313 multiplexes the DS3 bit streams 401 and 402 tocreate the multiplexed DS3 data bit stream 405 that is fed into the DS3deframer 500 along with the valid bit stream 407 and the channel bitstream 409. A dashed line 515 indicates a first pipe stage. In the firstpipe stage, a per-channel state memory 511 sends information to a synchunt per-alignment memory 513. The per-channel state memory 511 alsosends information to a register 521. Bits indicating the per-alignmentstate are transmitted from the sync hunt per-alignment memory 513 to aregister 523. Also in the first pipe stage, the data bit stream 409 isstored in a register 517 while the streams 405, 407 are stored in aregister 519. A dashed line 533 indicates a second pipe stage of the DS3deframer 500. In the second pipe stage, bits from the registers 517,519, 521 and 523 flow to a DS3 deframing logic 525 and a DS3 sync huntlogic 527. The bits flowing from the register 517 indicate side (i.e.,channel). In this example, the side information from the register 517indicates whether the bit stream from the register 519 is the DS3 bitstream 401 or the DS3 bit stream 402. Data from the register 521indicates a global state for the DS3 deframer and a counter valueindicating location within a subframe for a given DS3 signal. The globalstate is described later in more detail with reference to FIGS. 8A-8B.The bits from the register 523 indicate the per-alignment state. Outputfrom the DS3 sync hunt logic 527 flows into a set of registers 529, 531.The register 531 also receives input from the DS3 deframing logic 525.The bits stored in register 531 loop back into the per-channel statememory 511. The bits stored in the register 529 flow back into the synchunt per-alignment memory 513. Output from the DS3 deframing logic 525is also stored in a register 533 before flowing to a DS2 deframer.

FIG. 6 is a diagram of a DS2 deframer according to one embodiment of theinvention. The data bit stream 406, the validity bit stream 408 and thesubchannel bit stream 410 flow from the register 531 of the DS3 deframer500 to the DS2 deframer illustrated in FIG. 6. The bits stored in aregister 605 are from the subchannel bit stream 410 and the data bitstream 406. The bits stored in a register 603 are from the validity bitstream 408. A dashed line 635 indicates a first pipe stage of the DS2deframer. In the first pipe stage, bits flow from the register 603 tothe register 607 and from the register 605 to a register 609. Inaddition, the bits from the register 605 flow through a per-channelstate memory 623 and into a register 611. A dashed line 637 indicates asecond pipe stage of the DS2 deframer. In the second pipe stage, bitsstored in the registers 607, 609 and 611 flow into registers 613, 615and 617 respectively. The bits from the register 611 also flow through async hunt per-alignment memory 621 and into a register 619. A thirddashed line 639 indicates a third pipe stage for the DS2 deframer 322.The bits stored in the registers 613, 615, 617 and 619 flow into a DS2deframing logic 625 and a DS2 sync hunt logic 627. After being processedby the DS2 sync hunt logic 627, bits are stored in a register 633 beforeflowing back into the sync hunt per-alignment memory 621. Output fromboth the DS2 deframing logic 625 and the DS2 sync hunt logic 627 isstored in a register 631. From the register 631, bits flow back into theper-channel state memory 623. Output from the DS2 deframing logic 625also flows into a register 629 before continuing on to a DS1 deframer.

FIG. 7 is a diagram of a DS1 deframer according to one embodiment of theinvention. The data bit stream 412, validity bit stream 414, and thesubchannel bit stream 416 flow from a DS2 deframer 750 to a set ofregisters 701 and 703. The bits stored in the register 701 are from thevalidity bit stream 413. The bits stored in the register 703 are fromthe data bit stream 412 and the subchannel bit stream 416. A dashed line747 indicates a first pipe stage of the illustrated DS1 deframer. In thefirst pipe stage, bits from the registers 701 and 703 flow intoregisters 705 and 707 respectively. The bits from the register 703 alsoflow through a per-channel state memory 719 and into a register 709. Adashed line 745 indicates a second pipe stage of the DS1 deframer 324.The bits in the registers 705, 707 and 709 flow into a set of registers711, 713 and 715 respectively. The data stored in the register 709indicates a global state for the DS1 deframer and a counter indicatinglocation within a subframe for a given DS2 signal carried in the DS3signal. The DS1 global states and the counter are described later inmore detail with reference to FIGS. 14A-14B, 16A-16B, and 18. In thesecond pipe stage of the illustrated DS1 deframer, bits flow from a synchunt read buffer 721 to a register 717. A memory controller 761 storesbits indicating per-alignment state into a set of read FIFOs 737 of thesync hunt read buffer 721. A selector 739 of the sync hunt read buffer721 selects the per-alignment state bits stored in the set of FIFOs 737to be stored in the register 717. A dashed line 743 indicates a thirdpipe stage of the DS1 deframer 324. In the third pipe stage of the DS1deframer 324, bits from the registers 711, 713, 715 and 717 flow into aDS1 deframing logic 725 and a DS1 sync hunt logic 727. After processingby the DS1 deframing logic 725, the bits previously stored in theregisters 711 and 713 are stored in a register 729. After processing bythe sync hunt logic 727, bits from the register 715 are stored in aregister 731 before looping back to the per-channel state memory 719.The per-alignment state bits stored in the register 717 are processed bythe DS 1 sync hunt logic 727 and stored in a register 733. The bits inthe register 733 flow into a sync hunt write buffer 735. These bits arestored in a set of FIFOs 740 of the sync hunt write buffer 735 andaccessed by a selector 741, before being processed by the memorycontroller 761.

In one embodiment of the invention, each pair of DS1 subchannels (e.g.the pair of DS 1 subchannels DS1 subchannel 0 of DS3 side 0 and DS1subchannel 0 of DS3 side 1) has a read and write FIFO that is two 7 byteentries deep. This size provides space for 14 sync hunt states. Theentries in each FIFO provide enough latency tolerance to keep the synchunt logic working while the memory controller 321 serves otherchannels. Each one of the deframing slice's FIFOs are independentlywritable and readable. The memory controller 761 writes to the readFIFOs. The sync hunt core reads the read FIFOs. The sync hunt corewrites to the write FIFOs and the memory controller reads from the writeFIFOs. In one embodiment of the invention, the FIFOs are asynchronousbecause each DS3 bit stream may run at a different bit rate. In oneembodiment of the invention, each DS1 sync hunt begins by flushing theread and write FIFOs of any possible stale sync hunt data. The sync huntlogic then allows the read FIFO to become full. After the read FIFO isfull, sync hunting begins. The memory controller and asynchronous FIFOsensure sufficient provision of bandwidth to the DS1 sync hunt logic. Inanother embodiment of the invention, the read/write FIFOs are larger toaccommodate higher density deframing slices.

In one embodiment of the invention, the addressing pointers for the synchunt memory 323 (shown in FIG. 3) are stored in the FIFO core 721, 735.Placing the addressing pointers in the FIFO core 721, 735 reduces thecomplexity of resetting and controlling the addressing pointers. Inanother embodiment of the invention, the addressing pointers are storedin the memory controller 761. In one embodiment of the invention, whichstores the addressing pointer in the memory controller 761, a registerarray for each deframer slice is placed in a larger register array thatis placed in the memory controller 761. Such a design provides thebenefit of reducing the hardware necessary for implementing thedeframer.

As shown by FIGS. 5-7, each deframer performs both sync hunting anddeframing. Sync hunting is performed by the sync hunt logics 527, 627,and 727. After synchronization, a bit stream is deframed by thecorresponding one of the deframing logics 525, 625, 725 while the synchunt logic continues to monitor sync. If a channel gets out of sync,sync hunt for that channel is restarted.

In one embodiment of the invention, each deframer sync hunts half of thetotal number of channels because the sync hunt memory is shared.Limiting the sync hunting reduces the space necessary to implement thedeframer. In such an embodiment, each deframing slice can processchannels from two DS3 bit streams, but sync hunts one of those bitstreams. For example, the DS3 deframer sync hunts either the first DS3bit stream or the second DS3 bit stream, but not both at the same time.Deframing (i.e., identification of payload and overhead bits) isconducted for both channels simultaneously as it is not costly toimplement. The DS2 deframer sync hunts either a DS2 channel from thefirst DS3 bit stream or a DS2 channel from the second DS3 bit stream,but not DS2 channels from both DS3 bit streams. Similarly, the DS1deframer sync hunts DS1 channels from either the first DS2 bit stream orthe second DS3 bit stream.

Synchronizing a bit stream (sync hunting) comprises searching for a bitpattern formed by an alignment signal. For example, a DS3 frame includesseven subframes. Each subframe comprises eight 85 bit blocks. The firstbit of each block is an overhead bit which includes bits of thealignment signal. For a DS3 signal, the alignment signal includes F-bitsand M-bits. The F-bits or framing bits form a bit pattern “1001” in eachsubframe at blocks two, four, six, and eight. Each F-bit is separated by170 bits. The M-bits or multiframing bits form a bit pattern “010”. TheM-bits occur in the first block of the fifth, sixth, and seventhsubframe. It should be understood that the invention is not limited tothese bit patterns. In another embodiment of the invention, the logicsearches for different bit patterns to synchronize a bit stream orsignal. The sync hunting logic 527, 627 maintains multiple per-alignmentstate machines to be described. The sync hunt logic performs synchunting concurrently for multiple per-alignment state machines using asingle bit. The logic determines if the bit matches the F-bit patternfor one per-alignment state machine and the M-bit pattern for adifferent per-alignment state machine. The sync hunting is described inmore detail with reference to FIGS. 8A-12.

FIGS. 8A-8B are flow charts for DS3 sync hunting according to oneembodiment of the invention. FIG. 8A is a flow chart for DS3 synchunting according to one embodiment of the invention. As indicated inFIG. 5, bits are used from the registers 517, 519, 521 and 523. If a bitstored in the register 517 indicates invalidity, then a correspondingsignal bit stored in the register 519 is not processed by the followinglogic. The term signal bit is used to distinguish data bits of the databit stream from stuffing bits added to the data bit stream by thereceiving network element. The signal bits (data bits) can becategorized as payload bits or overhead bits. Although a signal bit maybe a payload bit from the perspective of the DS3 deframer, it may be anoverhead bit from the perspective of the DS2 or DS1 deframer. Thefollowing logic is performed for each subchannel or side.

At block 801 of FIG. 8A, a value X is reset. The value X represents theper-alignment state machine being used. At block 803, a signal bit fromthe register 519 is received and saved as a first framing F-bit in asubframe alignment shift register for a per-alignment state machine X.

At block 805, it is determined if X=N−1(N being the total number ofper-alignment state machines). This check determines if the logic hasiterated through all of the per-alignment state machines. If it isdetermined that X is not equal to N−1, then at block 807, X isincremented. From block 807, control flows back to block 803. If, atblock 805, it is determined that X=N−1, then at block 808 X is reset. Atblock 809, another signal bit is received from the register 519 andsaved as the second F-bit in the subframe alignment shift register for aper-alignment state machine X.

FIG. 9 illustrates an example of storing DS3 bits in per-alignment statemachines as potential framing bits according to one embodiment of theinvention. The example illustrated in FIG. 9 includes 170 per-alignmentstate machines for a DS3 signal. In FIG. 9, only four of the 170per-alignment state machines are shown. A DS3 bit stream 901 is receivedand stored as described in FIG. 8A. A bit 0 (the first bit) of the bitstream 901 is stored in F1 (first F-bit) of per-alignment state machine0 903 (the first per-alignment state machine). The next bit, bit 1, isstored as F1 in per-alignment state machine 1 905 (the secondper-alignment state machine). Bits 168 and 169 are stored as F1 inper-alignment state machines 168 908 and 169 909 respectively. The bits170 and 171 of the bit stream 901 are stored as F0 (the second F-bit) inthe per-alignment state machines 0 901 and 1 903. Bits 337 and 338 ofbit stream 901 are stored in the per-alignment state machines 168 907and 169 909 respectively as F0.

Returning to FIG. 8A, after this second F-bit is stored at block 809, async hunt state machine for the per-alignment state machine X is set toindicate state as “HUNTING_(—)010” at block 811. The per-alignment statemachine is described in Table 1.

TABLE 1 State Encoding for DS3 Sync Hunt State Bits (most significantbit to least significant bit) State Machine Action 0 - H1 H0 - F1 F0Shift0 Shift in potential first F-bit into F0 0 - H1 H0 - F1 F0 Shift1Shift F0 to F1 and shift potential second bit into F0 0 - H1 H0 - F1 F0Hunt0, Hunt1 + Freeze {F1, F0}, use to check incoming potential F-bits +Shift incoming potential M-bits into {H1, H0} shift register. If {H1,H0, incoming bit} = 010, then go to Maintain_010 state and set {S2, S1,S0} = X1, else if 10 subframes have passed, then fail, else continueHunt0, Hunt1 state 1 S2 S1 S0 P F1 F0 Maintain_010 + continue to use{F1, F0} to check incoming potential F-bits + {S2, S1, S0} forms a statemachine to check incoming potential M-bits. Remember previous potentialM-bit using P, and use to check that potential framing bits matchpatterns X1=X2, P1=P2. 1 1 1 1 - - - Fail

At block 813 it is determined if X=N−1. If X does not equal N−1, then atblock 815 X is incremented. From block 815 control flows back to block809. If it is determined at block 813 that X equals N−1, then at block817 X is reset. At block 819 a signal bit is received from the register519. From block 819, control flows to both blocks 821 and 823. At block821, framing bit pattern verification for a per-alignment state machine(X+85) MOD 170 is performed concurrently with verification of F-bits fora per-alignment state machine X at block 823.

At block 823, it is determined if the bit received at block 819 is thenext expected F-bit for a per-alignment state machine X. If the receivedbit is the next expected F-bit for the per-alignment state machine X,then at block 831 it is determined if X=N−1. If at block 823 it isdetermined that the bit is not the next expected F-bit for theper-alignment state machine X, then at block 827 the sync hunt statemachine for the per-alignment state machine X is set to indicatefailure. From block 827 control flows to block 831. If X does not equalN−1, then at block 829 X is incremented. From block 829 control flowsback to the block 819. If it is determined at block 831 that X equalsN−1, then at block 833 it is determined if all per alignment statemachines have failed or a time out has occurred. If all of theper-alignment state machines have failed or a timeout has occurred, thenat block 835 the DS3 sync hunting restarts. In an alternative embodimentof the invention, a timeout forces the sync hunt logic to select one ofthe per-alignment state machines which have not failed. If it isdetermined at block 833 that all of the per-alignment state machineshave not failed and a timeout has not occurred, then at block 837 it isdetermined if only one per-alignment state machine remains valid. If itis determined at block 837 that more than one per-alignment statemachine still remains valid, then control flows to block 817. If onlyone per-alignment state machine remains valid, then it is determined ifthe per-alignment state machine indicates a state of “MAINTAIN_(—)010”at block 838. If it is determined that the per-alignment state machineindicates “MAINTAIN_(—)010”, then the DS3 framing pattern has beendetected and at block 839 DS2 deframing begins. If it is determined atblock 838 that the per-alignment state machines does not indicate“MAINTAIN_(—)010”, then control flows to block 817.

FIG. 8B is a flow chart for performing block 821 of FIG. 8A according toone embodiment of the invention. From block 819 of FIG. 8A control flowsto a block 843. At block 843, it is determined if the received bit isbit 0 for a subframe of a per-alignment state machine (X+85) MOD 170. Ifthe received bit is not bit 0 for a subframe of this per-alignment statemachine, then control flows back to block 831 of FIG. 8A. If it isdetermined at block 843 that the received bit is a bit 0, then at block845 it is determined if the sync hunt state machine for thisper-alignment state machine indicates a state of “MAINTAIN_(—)010”. Ifthe sync hunt state machine for this per-alignment state machineindicates “MAINTAIN_(—)010”, then at block 849 it is determined if thereceived bit is the correct bit in accordance with the state indicatedby the sync hunt state machine. The states represented by the sync huntstate machine s2-s0 are shown in table 2.

TABLE 2 States Represented by Sync Hunt State Machine s2 s1 s0 statename 0 S0 1 S01 2 S010 3 SX1 4 SX2 5 SP1 6 SP2 7 FAIL

Each state represents a transitions state. The state S0 means the synchunt logic is looking for the bit 0. The state S01 means the sync huntstate machine has stored a previous bit 0 and is looking for a bit 1.The state S010 means the sync hunt logic saw a 1 as the previous bit andis looking for a 0. The state SX1 represents a transition in the synchunt to looking for the first X framing bit. The state SX2 means thesync hunt logic is hunting for the second X framing bit that should bethe same as the P bit shown in table 1 (the first X framing bit). Thestate SP1 represents a transition in the sync hunt to looking for thefirst P framing bit. The state SP2 means the sync hunt logic is huntingfor the second P framing bit that should be the same as the P bit shownin table 1 (first P framing bit).

If the received bit is not the correct bit in accordance with theindicated state, then at block 851 the sync hunt state machine for thisper-alignment state machine is set to indicate a state of fail. Fromblock 851, control flows back to block 831 of FIG. 8A. If at block 849it is determined that the received bit is the correct bit in accordancewith the sync hunt state machine, then at block 853 the bit is shiftedinto the sync hunt state machine for this per-alignment state machine,thus updating the state. From block 853, control flows back to block 831of FIG. 8A. If at block 845, it is determined that the sync hunt statemachine for this per-alignment state machine does not indicate state as“MAINTAIN_(—)010”, then at block 855 it is determined if the M-bitsstored in the sync hunt state machine and the received bit form a bitpattern “010”. If all of these bits form the bit pattern “010”, then atblock 859 the sync hunt state machine of this per-alignment statemachine is set to indicate maintenance state or “MAINTAIN_(—)010”.Control flows from block 859 to block 831 in FIG. 8A. If at block 855 itis determined that the framing M-bits stored in the sync hunt statemachine and the received bit do not form the bit pattern “010”, then atblock 857 the received bit is shifted into the sync hunt state machineas H0 and the bit stored as H0 is shifted into H1 of the sync hunt statemachine. At block 861 it is determined if nine subframes have passedbased on the counter and global state bits from the register 521 of FIG.5. If nine frames have not passed for this per-alignment state machine,then control flows to block 831 of FIG. 8A. If it is determined at block861 that nine subframes have passed for the per-alignment state machine,then at block 863 the sync hunt state machine for this per-alignmentstate machine is set to indicate a state of fail. From block 863,control flows to block 831 of FIG. 8A.

A DS3 master state machine controls the DS3 per-alignment statemachines. The DS3 master state machine initializes and maintains theper-alignment state machines. The states of the DS3 master state machineare described in Table 3 below.

TABLE 3 DS3 Master States SH3_IDLE channel is idle SH3_SHIFT_SF0 firstsubchannel of hunt - used to reset states SH3_SHIFT_SF1 secondsubchannel of hunt SH3_HUNT0 hunting SH3_HUNT1 hunting, after 10 framesSH3_WAIT_WINNER found a single winner, but now wait for winner again;the purpose is to simplify the counter logic in the DS3 deframers

The states of the master state machine can be represented with a varietyof values. In one embodiment of the invention, a S113_WAIT_WINNER stateis not maintained.

FIG. 10 is a diagram illustrating organization of the per-alignmentstate machines in the sync hunt per-alignment memory 513 of FIG. 5according to one embodiment of the invention. In FIG. 10, theper-alignment state machines are arranged as two columns of 85per-alignment state machines. This organization of the per-alignmentstate machines allows the use of a single port register array instead ofa dual port register array. This organization also allows the sync huntlogic to accomplish 2 tasks concurrently: both the task of verifyingsubframe alignment with F-bit patterns for a per-alignment state machineX and the task of verifying framing bit patterns for a per-alignmentstate machine (X+85) MOD 170. In one embodiment, each per-alignmentstate machine is 7 bits wide. In another embodiment, each per-alignmentstate machine is wider.

In the DS2 format, a DS2 frame is comprised of four subframes. Eachsubframe includes six 49 bit blocks. Each block includes an overhead bitfollowed by 48 bits. An M-bit is the overhead bit for the first block ofeach subframe. The M-bits form either the bit pattern “0111” or “0110”in a given DS2 frame. An F-bit is the overhead bit for blocks three andsix of each subframe. The two F-bits of a subframe form the bit pattern“01” in each subframe. The DS2 alignment bit patterns are used to aid inthe understanding of the invention and not as limitations upon theinvention.

FIGS. 11A-11B are flow charts for DS2 sync hunting according to oneembodiment of the invention. FIG. 11A is a flow chart for performing DS2synchronization hunting according to one embodiment of the invention. Ifa valid bit from the register 613 indicates invalidity for acorresponding signal bit from the register 615, the following logic isnot performed on the invalid signal bit. At block 1101, a value X isinitialized. Again, the value X represents a per-alignment state machinebeing used. At block 1103, a signal bit from the register 615 isreceived and saved as a first framing F-bit in a subframe alignmentshift register of a per-alignment state machine X. At block 1105, thesync hunt state machine for the per-alignment state machine X is set toindicate “HUNTING_(—)01”.

FIG. 12 illustrates an example of storing bits in DS2 per-alignmentstate machines as potential alignment bits according to one embodimentof the invention. In FIG. 12, a bit stream 1201 is received. In thisexample, there are 147 per-alignment state machines, but only sixper-alignment state machines are shown. Bits 0, 1, and 2 of the bitsstream 1201 are stored as P0 in per-alignment state machines 1203, 1205,and 1207 respectively. Bits 144, 145 and 146 are stored as P0 inper-alignment state machines 1209, 1211, and 1213 respectively.

Returning to FIG. 11A at block 1107, it is determined if X is equal toN−1. If X is not equal to N−1, then at block 1109 X is incremented. Fromblock 1109, control flows back to block 1103. If at block 1107 it isdetermined that X does equal N−1, then at block 1111 X is reset. Atblock 1113, another signal bit is received from the register 615. Fromblock 1113 control flows to both blocks 1115 and 1117. At block 1115 itis determined if the received bit is the next expected F-bit for aper-alignment state machine X in concurrence with verification of M-bitpatterns for a per-alignment state machine (X+98) MOD 147 at block 1117.In other words, the search for valid F-bit and M-bit patterns areperformed concurrently. If the received bit is the next expected F-bitfor the per-alignment state machine X, then at block 1123 it isdetermined if X equals N−1. If at block 1115 it is determined that thereceived bit is not the next expected F-bit for the per-alignment statemachine X, then at block 1119 the sync hunt state machine for theper-alignment state machine X is set to indicate a state of fail. Forexample, if per-alignment state machine is expecting a 1 but receives a0, then synchronization represented by that state machine cannot becorrect. From blocks 1119 and 1117 control flows to block 1123. If it isdetermined at block 1123 that X does not equal N−1, then at block 1121 Xis incremented. From block 1121 control flows to block 1113. If at block1123 it is determined that X equals N−1, then it is determined if allthe per-alignment state machines have failed or a time out has occurredat block 1125. If all of the per-alignment state machines have failed ora time out has occurred, then at block 1127 DS2 sync hunting isrestarted. In an alternative embodiment, a timeout forces the sync huntlogic to select one of the remaining per-alignment state machines as thewinner. If it is determined at block 1125 that all the state machineshave not failed or a time out has not occurred, then at block 1129 it isdetermined if only one per-alignment state machine has not failed. If itis determined at block 1129 that more than one per-alignment statemachine remains valid, then control flows to block 1111. If only oneper-alignment state machine has not failed, then it is determined atblock 1130 if the per-alignment state machine indicates a state“MAINTAIN_(—)01”. If the valid per-alignment state machine does notindicate the state “MAINTAIN_(—)01”, then control flows to block 1111.If the valid per-alignment state machine indicates the state“MAINTAIN_(—)01”, then the DS2 stream has been synchronized and DS2deframing begins at block 1131.

FIG. 11B is a flow chart for performing block 1117 of FIG. 11A accordingto one embodiment of the invention. At block 1133 it is determined ifthe bit received at block 1113 is bit 0 of a subframe for aper-alignment state machine (X+98) mod 147 using the counter bits andstored F-bits from the register 617. If the received bit is not bit 0,then control flows to block 1123 of FIG. 11A. If it is determined atblock 1133 that the received bit is bit 0 of a subframe for thisper-alignment state machine, then at block 1135 it is determined if thestate indicated by the sync hunt state machine for this per-alignmentstate machine (bits from the register 619) is “MAINTAIN_(—)01”. Table 4describes a DS-2 per-alignment state machine.

TABLE 4 State Encoding for DS2 Sync Hunt State Bits (most significantbit to least significant bit) State Machine Action 0 - H1 H0 - F0 Shift0Shift in potential first F-bit into F0 0 - H0 - F0 Hunt0, Hunt1 + Freeze{F0}, use to check incoming potential F-bits + Shift incoming potentialM-bits into {H0} shift register. If {H0, incoming bit} = 01, then go toMaintain_01 state and set {S2, S1, S0} = S01, else if 8 subframes havepassed, then fail, else continue Hunt0, Hunt1 state 1 S2 S1 S0 F0Maintain_01 + continue to use {F0} to check incoming potential F-bits +{S2, S1, S0} forms a state machine to check incoming potential M-bits.Check that incoming potential framing bits match pattern 011X, where Xcan be either 0 or 1. 1 1 1 1 Fail

If the sync hunt state machine of the per-alignment state machineindicates “MAINTAIN_(—)01”, then at block 1137 it is determined if thereceived bit is the correct bit in accordance with a state indicated bythe sync hunt state machine as shown in table 5.

TABLE 5 States Represented by Synch Hunt State Machine s2 s1 s0 statename 0 S0 1 S01 2 S011 3 X 7 FAILAs with the DS3 sync hunt state machine, each state represents atransition state. The state S0 means the sync hunt logic is looking fora bit 0. The state S01 means the sync hunt state machine has seen aprevious bit 0 and is looking for a bit 1. The state S011 means the synchunt logic has seen a bit 1 as the previous bit and is looking for a 1.The state X represents acceptance of any bit since the fourth M framingbit can be either a 0 or 1.

If the received bit is not the correct bit, then at block 1139 the synchunt state machine is set to indicate a state of fail. From block 1139control flows back to block 1123 of FIG. 11A. If at block 1137 it isdetermined that the received bit is correct, then at block 1141 the synchunt state machine of this per-alignment state machine is updated. Forexample, the sync hunt state machine indicated the state S0 and thereceived signal bit is a 1, then state is updated to S01. From block1141, control flows back to block 1123 of FIG. 11A. If at block 1135 itis determined that the sync hunt state machine does not indicate a stateof “MAINTAIN_(—)01”, then at block 1143 it is determined if a framingM-bit stored in the sync hunt state machine and the received bit form abit pattern “01”. If these bits form this bit pattern, then at block1145 the sync hunt state machine is set to indicate a maintenance stateor “MAINTAIN_(—)01”. From block 1145, control flows back to block 1123of FIG. 11A. If it is determined at block 1143 that the stored M-bit andthe received bit do not form the pattern “010”, then at block 1147 thereceived bit is shifted into the sync hunt state machine as the M-bit(110). At block 1149 it is determined if seven subframes have passedbased on the counter and global state bits from the register 617 of FIG.5. If seven frames have not passed for this per-alignment state machine,then control flows to block 1123 of FIG. 11A. If seven subframes havepassed for this per-alignment state machine, then at block 1151 the synchunt state machine for this per-alignment state machine is set toindicate a state of fail. From block 1151, control flows to block 1123of FIG. 11A.

Similar to the DS3 deframer, a master state machine controls the DS2sync hunt logic. The states of the DS2 master state machine are shown inTable 6 below.

TABLE 6 DS2 Master States SH2_IDLE channel is idle SH2_SHIFT_SF firstsubchannel of hunt - used to reset states SH2_HUNT huntingSH2_WAIT_WINNER found a single winner, but now wait for winner again;the purpose is to simplify the counter logic in the DS2 deframersSH2_RUN_FIRST first bit of deframing, used to send signal downstreamSH2_RUN steady state of run, continues to monitor framing SH2_RPT_FAILfailed to find any good frame alignment or failed to find a singlewinner after a timeout

FIG. 13 is a diagram illustrating organization of the per-alignmentstate machines in the sync hunt per-alignment memory 621 of FIG. 6according to one embodiment of the invention. In FIG. 13, theper-alignment state machines are arranged as three columns of 49per-alignment state machines. This organization of the per-alignmentstate machines allows the sync hunt logic to accomplish 2 tasksconcurrently: both the task of verifying subframe alignment with F-bitpatterns for a per-alignment state machine X and the task of performingthe sync hunt for a per-alignment state machine (X+98) MOD 147. Eachper-alignment state machine is 5 bits wide. In another embodiment of theinvention, the per-alignment state machines are wider.

The alignment signal for the DS1 superframe format is formed byinterleaving framing and superframing bits. The first bit of each DS1frame is a bit for the alignment signal. The interleaved framing andsuperframing bits form the pattern “100011011100”. The DS1 superframealignment bit patterns are used to aid in the understanding of theinvention and not as limitations upon the invention.

FIGS. 14A-14B are flow charts for DS1 super frame sync hunting performedby the DS1 sync hunt logic 727 of FIG. 7 according to one embodiment ofthe invention. FIG. 14A is the flow chart for initializing theper-alignment state machines for DS1 super frame sync hunting accordingto one embodiment of the invention. As with the DS3 and DS2 synchunting, the following logic is not performed on a signal bit from theregister 713 if a corresponding validity from the register 711 indicatesthe signal bit as invalid. At block 1401, a value Y is reset. The valueY is a counter variable for the number of bits seen for eachper-alignment state machine. At block 1403, a value X is reset. At block1405 a signal bit is received from the register 713 and stored in aper-alignment state machine X [Y]. At block 1407 it is determined ifX=N−1. If X does not equal N−1, then at block 1411 X is incremented.From block 1411, control flows back to block 1405. If it is determinedat block 1407 that X=N−1, then at block 1409 it is determined if Yequals three. If Y is not equal to three, then at block 1413 Y isincremented. From block 1413 control flows to block 1403. If Y doesequal three, then framing verification is performed at block 1415.

FIG. 15 is an exemplary illustration of storing bits in DS1per-alignment state machines according to one embodiment of theinvention. In this example of DS1 super frame sync hunting, 192per-alignment state machines are maintained. If a bit 0 is the first bitof a bit stream, then bit x (x being any number from 0 to 766) will bestored in a per-alignment state machine x MOD 192 in position x DIV 192.In FIG. 15, a bit stream 1501 is received. Bits 0, 192, 384, and 576 arestored as S3, S2, S1 and S0 respectively of the per-alignment statemachine 1503 (per-alignment state machine 0). Bits 191, 383, 575, and766 are stored as S3, S2, S1 and S0 respectively of the per-alignmentstate machine 1505 (per-alignment state machine 191). All of the first767 bits are stored in the 192 per-alignment state machines.

FIG. 14B is a flow chart for performing block 1415 of FIG. 14A accordingto one embodiment of the invention. At block 1417 each per-alignmentstate machine with a bit sequence matching an illegal bit sequence isupdated to indicate a state of fail. Table 7 shows the illegal 4 bitsequences.

TABLE 7 Illegal bit sequences for DS1 super frame Illegal 4 bitsequences 0000 0101 1010 1111The bit sequences identified in table 7 do not occur in the framing bitstream for DS1 super frame formatting.

At block 1419 a signal bit is received from the register 713. At block1421 it is determined if the received bit is the expected bit inaccordance with the indicated state of the per-alignment state machine Xas shown in Table 8 below.

TABLE 8 States Represented by Synch Hunt State Machine State Name statemachine encoding S1 0x1 S2 0x2 S3 0x3 S4 0x4 S6 0x6 S7 0x7 S8 0x8 S9 0x9Sb 0xb Sc 0xc Sd 0xd Se 0xe FAIL 0xf BAD_0 0x0 BAD_1 0x5 BAD_2 0xaThese states are based on the super frame framing bit stream100011011100.

If the received bit is not the expected bit, then at block 1423 theper-alignment alignment state machine is updated to indicate a state offail. If the received bit is the expected bit, then at block 1431 theper-alignment state machine is updated. Control flows, from block 1423and block 1431 to block 1437. At block 1437 it is determined if X=N−1.If X does not equal N−1, then at block 1439 X incremented. Control flowsfrom 1439 to block 1419. If at block 1437 it is determined that X equalsN−1, then at block 1425 it is determined if all per-alignment statemachines have failed or a timeout has occurred. If all state machineshave not failed and a timeout has not occurred, than at block 1427 it isdetermined if only one state machine remains valid. If, at block 1425,it is determined that all of the per-alignment state machines havefailed, then at block 1435 DS1 super frame sync hunting is restarted. Ifit is determined at block 1427 that only one state machine remainsvalid, then synchronization has been found for the DS1 signal and atblock 1429 DS1 super frame deframing begins. If it is determined atblock 1427 that more than one per-alignment state machine is valid, thenat block 1441 X is reset and control flows back to 1419.

The alignment signal for the DS1 extended superframe format comprisesframing bits (F-bits) positioned at the beginning of every block that isa multiple of four (i.e., the first bit of blocks 4, 8, 12, 16, etc).The F-bits form the pattern “001011” over 24 frames. The pattern isrepeated every 24 frames. The DS1 extended superframe alignment bitpatterns are meant to aid in the understanding of the invention and notas limitations upon the invention.

FIGS. 16A-16B are flow charts for sync hunting a DS1 extended superframe signal performed by the DS1 sync hunt logic 727 of FIG. 7according to one embodiment of the invention. FIG. 16A is a flow chartfor DS1 extended super frame sync hunting according to one embodiment ofthe invention. At block 1601 a value X is reset. At block 1603 a signalbit is received from the register 713 and shifted into a per-alignmentstate machine X. Table 9 describes a DS1 Extended Superframeper-alignment state machine.

TABLE 9 State Encoding for DS1 Extended Super Frame Sync Hunt State Bits(most significant bit to least significant bit) State Machine Action 0 -H1 H0 Shift0 Shift in potential first framing but into H0 0 - H1 H0Shift1 Shift H0 to H1 and shift potential second bit into H0 0 - H1 H0Hunt0, Hunt1 + Shift incoming potential framing bits into {H0} shiftregister. If {H1, H0, incoming bit} = 101, then go to Maintain_101 stateand set {S2, S1, S0} = S00101, else if 8 bits have passed for thisper-alignment state machine, then fail, else continue Hunt0, Hunt1 state1 S2 S1 S0 Maintain_01 + {S2, S1, S0} forms a state machine to checkincoming potential framing bits. Use {S2, S1, S0} to check that incomingpotential framing bits match expected pattern. 1 1 1 1 FailAt block 1605, it is determined if X=N-1. If X is not equal to N-1, thenat block 1607 X is incremented. From block 1607, control flows to block1603. If it is determined at block 1605 that X=N-1, then at block 1609 abit is received and stored as a second F-bit in the per-alignment statemachine X[O]. An illustration of storing F-bits in the per-alignmentstate machines is described with reference to FIG. 17.

FIG. 17 is an exemplary illustration for storing F-bits in per-alignmentstate machines for sync hunting DS1 extended superframe according to oneembodiment of the invention. In this example 772 per-alignment statemachines are maintained, but only four are shown. In FIG. 17, a bitstream 1701 is received. Bits 0 and 772 are stored as F1 and F0respectively of a per-alignment state machine 1703. Bits 6 and 778 arestored as F1 and F0 respectively of a per-alignment state machine 1705.Bits 771 and 1543 are stored as F1 and F0 respectively of aper-alignment state machine 1707. Every bit of the first 1544 valid bitsin the bit stream 1701 will be stored in the per-alignment statemachines. We will return to describing FIG. 16A.

At block 1611, the per-alignment state machine X is set to indicate“HUNTING_(—)101”. At block 1613 it is determined if X=N−1. If X is notequal to N−1, then at block 1615, X is incremented. Control flows backto block 1609 from block 1615. If it is determined at block 1613 that Xis equal to N−1, then at block 1617 X is reset. At block 1619 anothersignal bit is received from the register 713. At block 1621 frame bitverification is performed. At block 1625 it is determined if X=N−1. If Xis not equal to N−1, then at block 1623 X is incremented. From block1623 control flows to block 1619. If X is equal to N−1, then at block1627 it is determined if all per-alignment state machines have failed ortimeout has occurred. In an alternative embodiment of the invention, atimeout forces the sync hunt logic to select one of the validper-alignment state machines as a winner. If all state machines havefailed or timeout has occurred, then at block 1629 DS1 sync hunting isrestarted. If it is determined at block 1627 that all per-alignmentstate machines have not failed or a timeout has not occurred, then atblock 1631 it is determined if only one per-alignment state machineremains valid. If more than one per-alignment state machine remainsvalid, then control flows to block 1617. If only one per-alignment statemachine remains valid, then it is determined at block 1632 if the validper-alignment state machine indicates a state “MAINTAIN_(—)101”. If thevalid per-alignment state machines does not indicate this state, thencontrol flows to block 1617. If it is determined at block 1632 that thevalid per-alignment state machine indicates the state “MAINTAIN_(—)101”,then synchronization has been found for the DS1 extended superframesignal and at block 1633 DS1 extended super frame deframing begins.

FIG. 16B is a flow chart for performing block 1621 of FIG. 16A accordingone embodiment of the invention. At block 1651 it is determined if thestate of the per-alignment state machine X is set to “MAINTAIN_(—)101”.If the state of the per-alignment state machine X is set to the state“MAINTAIN 101”, then at block 1633 it is determined if the received bitis the correct bit in accordance with the state indicated by theper-alignment state machine X. The states of the extended super frameDS1 states are shown in Table 10.

TABLE 10 States Represented by Synch Hunt State Machine s2 s1 s0 statename 0 S0 1 S00 2 S001 3 S0010 4 S00101 5 S001011 7 FAILEach state represents a transition state for DS1 extended super framesync hunting. The state S0 means the sync hunt logic is looking for thebit 0. The state S00 means the sync hunt state machine has stored aprevious bit 0 and is looking for a bit 0. The state S001 means the synchunt logic saw a 0 as the previous bit and is looking for a 1. The stateS0010 indicates that the sync hunt state machine has stored a previousbit 1 and is looking for a bit 0. The state S00101 means the sync huntstate machine has stored a previous bit 0 and is looking for a bit 1.The state S001011 indicates that the sync hunt state machine has storeda previous bit 1 and is looking for a bit 1. If the received bit is thecorrect bit, then the bit is shifted into the sync hunt state machine atblock 1655. Control flows from block 1655 to block 1623 of FIG. 16A. Ifthe received bit is not the correct bit in accordance with the stateindicated by the sync hunt state machine of the per-alignment statemachine X, then the per-alignment state machine X is set to indicate astate of fail at block 1657. Control flows from block 1657 to block 1623of FIG. 16A. If it is determined at block 1651 that the state of theper-alignment state machine X does not indicate “MAINTAIN_(—)101”, thenat block 1659 it is determined if the framing bits stored in theper-alignment state machine X and the received bit form the bit sequence“101”. If these bits form the bit sequence “101”, then at block 1661 thestate of the per-alignment state machine X is set to indicate amaintenance state of “MAINTAIN_(—)101”. Control flows from block 1661 toblock 1623 of FIG. 16A. If it is determined at block 1659 that thestored framing bits and the received bit do not form the bit pattern“101”, then at block 1663 it is determined if eight bits have been seenfor the per-alignment state machine X. If eight bits have been seen forthis per-alignment state machine, then at block 1665 the per-alignmentstate machine X is set to a state of fail. Control flows from block 1665to block 1623 of FIG. 16A. If it is determined at block 1663 that eightbits have not been seen for the per-alignment state machine X, then atblock 1667 the received bit is shifted into the per-alignment statemachine. From block 1667 control flows to block 1623 of FIG. 16A.

A master state machine regardless of extended super frame or super frameformatting controls DS1 sync hunting. The states of the master statemachine are shown in table 11.

TABLE 11 DS1 Master States SH1_IDLE channel is idle SH1_HUNT_FIRST firstsubchannel of hunt - used to reset states; when DS1 superframe thisstate lasts for 772 bits so that each of the 193 states can shift in 4frames per superframe SH1_HUNT hunting SH1_FIND_WINNER found a singlewinner, but now wait for winner again; the purpose is to simplify thecounter logic in the DS1 deframers SH1_RUN_FIRST first bit of deframing,used to send signal downstream SH1_RUN steady state of run, continues tomonitor framing

FIG. 18 is a diagram illustrating the organization of per-alignmentstate machines in the memory unit 321 of FIG. 3 according to oneembodiment of the invention. Although the sync hunt logic 727 of FIG. 7only uses 193 per-alignment state machines for DS1 super frame synchunting, the memory unit 321 of FIG. 3 is of a size sufficient to store770 of the 772 per-alignment state machines for DS1 extended super framesync hunting. In FIG. 18, the DS1 per-alignment state machines areorganized as 18480 rows of 7 per-alignment state machines. (18480 is theproduct of 6 DS3 pairs*28 DS1 subchannels*110 rows of 7 per-alignmentstate machines). Each of the per-alignment state machines are 4 bitswide. The two per-alignment state machines that are not stored in thememory unit 321 of FIG. 3 are located on chip with the DS1 deframingunit 209 of FIG. 2. Since the external memory unit in this example is 28bits wide, a total of 110+2/7 memory lines are needed for sync huntingDS1 extended super frames. Storing the 2/7 memory line in on-chip memorymakes the memory organization and bandwidth supplied by the memorycontroller uniform. In another embodiment of the invention, the memoryunit is expanded to accommodate the 2/7 memory line. In such anembodiment, the depth of the read/write FIFOs is increased toaccommodate a periodic dip in memory bandwidth supplied by the memorycontroller.

FIG. 19 is a flowchart for DS3 deframing performed by a DS3 deframinglogic according to one embodiment of the invention. Reference is made toFIG. 5 to help illustrate DS3 deframing. A bit stream is received atblock 1901 and a bit stream is received at block 1903. At block 1905 itis determined if each bit of the bit stream received at block 1901 isvalid (i.e. determine if receiving buffers are empty). At block 1907 itis determined if each of the signal bits stored in the register 519received at block 1903 is valid (i.e. determine if receiving buffers areempty). At block 1909, a bit is generated to indicate invalidity for anyof the bits of the bit stream if determined not to be valid at block1905. Similarly, at block 1911, a bit is generated to indicateinvalidity for any of the bits of the bit stream received at block 1903if determined not to be valid at block 1907. For each of the bits of thebit stream received at block 1901 determined to be valid at block 1905,a bit is generated to indicate validity at block 1910. Likewise, foreach of the bits of the bit stream received at block 1903 determined tobe valid at block 1907, a bit is generated to indicate validity at block1912. Control flows from blocks 1909-1912 to block 1915. At block 1915,the bit streams and validity bits are multiplexed. In addition, at block1915 a bit (channel bit) is generated for each bit during multiplexingto distinguish bit streams. At block 1919, for each bit of the originalbit streams, it is determined if the bit is an overhead bit. For eachbit determined to be an overhead bit, a bit is generated to identify thebit as a DS3 overhead bit at block 1921. At block 1931, bits are passedto a DS2 deframer. For each bit determined not to be an overhead bit atblock 1919, it is determined if each bit is valid at block 1923. A bitis generated at block 1925 to indicate invalidity for each invalid bit.Control flows from block 1925 to block 1931. For each of the bitsdetermined to be valid at block 1923, bits are generated to indicatevalidity and bit type as payload at block 1927. At block 1929, bits aregenerated to indicate a subchannel for each bit (i.e., indicate whichDS2 signal the bit is from). From block 1929, control flows to block1931.

FIG. 20 is a flowchart for DS2 deframing performed by a DS2 deframinglogic according to one embodiment of the invention. Reference is made toFIG. 6 to help illustrate. At block 2001, it is determined if a signalbit received from the register 615 of FIG. 6 is an overhead bit. If itis an overhead bit, then at block 2003 a bit is generated to indicatethe bit is a DS2 overhead bit. At block 2013, the bit is passed to theDS1 deframer. If it is determined at block 2001 that the bit receivedfrom the DS3 deframer is not a DS2 overhead bit, then at block 2005 itis determined if the bit is valid. If the bit is not valid, then a bitis generated to indicate invalidity of the bit at block 2007. From block2007, control flows to block 2013. If at block 2005 it is determinedthat the bit is valid, then at block 2009 bits are generated to indicatevalidity of the bit the type of the bit as payload. At block 2011, bitsare generated to indicate a subchannel for the bit (i.e., indicate whichDS1 signal the bit is from). Control flows from block 2011 to block2013.

FIG. 21 is a flowchart for DS1 deframing performed by a DS1 deframinglogic according to one embodiment of the invention. At block 2101 it isdetermined if a signal bit received from the register 713 of FIG. 7 isan overhead bit. If the bit is an overhead bit, then at block 2103 a bitis generated to identify the bit as a DS1 overhead bit. At block 2107the bit and all stuffing bits for the bit are passed to the protocolengine. If at block 2101 it is determined that the bit is not anoverhead bit, then at block 2105 the DS1 subchannel bits for the signalbit is replaced with a different DS1 subchannel bits. The initial DS1subchannel bits indicated whether the DS1 bit belonged to a DS1subchannel between 0 and 27, but the DS1 deframer 324 is processing 56DS1 subchannels. The new DS1 subchannels bits indicate which of the 56DS1 subchannels a given bit belongs. Control flows from block 2105 toblock 2107.

As described above, each successive deframer tags the multiplexed bitstream with successively more information. For example, the DS3 deframertags the bit stream with DS2 subchannel numbers and an indication of theDS3 channel (i.e., even or odd DS3 input). The DS2 deframer addsindicator bits indicating DS1 subchannels. The DS1 deframer extracts theFacility Data Link Channel and tags it, creating a new data link channelfor every data channel. The DS3 deframer tags bits to distinguishoverhead bits from information bits. A stuffing bit identifying a bit asa DS3 information bit is replaced by the DS2 deframer with a stuffingbit identifying the bit as a DS2 overhead bit or DS2 information bit.The same is done by the DS1 deframer.

FIG. 22 is a flowchart for change of frame alignment feed forwardingaccording to one embodiment of the invention. In FIG. 22, a bit streamis received at block 2201. At block 2203, DS3 sync hunting is performed.At block 2205 it is determined if the DS3 bit stream has beensynchronized. If the DS3 bit stream has not been synchronized, thencontrol flows back to block 2203. If the DS3 bit stream has beensynchronized, then at block 2207 the DS2 sync hunt mechanism is signaledby the DS3 sync hunt mechanism and DS3 deframing begins at block 2211.In response to the signal, the DS2 sync hunt is reset at block 2209while the DS3 mechanism begins to present deframed bits to the DS2 logicat block 2213. Until the bit stream terminates, control loops back toblock 2211 from block 2213. At block 2215, DS2 sync hunting isperformed. From block 2215, control flows to block 2217 where it isdetermined if the DS2 sync has been found. If the sync has not beenfound, then control flows back to block 2215. If the DS2 sync has beenfound, then at block 2219 the DS1 sync hunt mechanism is signaled by theDS2 sync hunt mechanism and DS2 deframing begins at block 2221. Inresponse to the signal, the DS1 sync hunt is reset at block 2223 whilethe DS2 mechanism begins to present deframed bits to the DS1 logic atblock 2222. Control loops back from block 2222 to block 2221 until thebit stream terminates. At block 2225, DS1 sync hunting is performed. Atblock 2227 it is determined if the DS1 sync has been found. If the DS1sync has not been found, then control flows back to block 2225. If theDS1 sync has been found, then at block 2229 DS1 deframing is performed.At block 2231, the deframed bits are presented to the protocol engine.Control loops back from block 2231 to block 2229 until the bit streamterminates.

Change of frame alignment feed forwarding increases the efficiency ofdeframing. As soon as the DS3 deframer 320 finds the DS3 alignment andbegins deframing, the DS2 deframer 322 will begin sync hunting for DS2alignment. The DS2 deframer will not look at every bit from the DS3deframer, though. The DS3 deframing logic is stuffing overhead bits,thus enabling the DS2 deframer to ignore bits which are not part of theDS2 alignment signal. Likewise, as soon as the DS2 deframer 322 findsthe DS2 alignment and begins deframing, the DS1 deframer 324 will beginsync hunting for DS1 alignment. The accelerated sync hunting enabled bychange of frame alignment forwarding provides the time for sharing synchunting memory.

The deframing logic described herein enables the production of networkelements with a high density of deframers. The validity bits used forsynchronizing bit streams reduces cost and complexity of a networkelement to process a large number of bit streams. The density or numberof bit streams that can be processed is not hindered by the generationof individual clocks for each channel or subchannel. Typically, thenumber of clocks increases linearly with the number of subchannels to beprocessed. Using the deframing logic described herein, the clock speedincreases with the density of bit streams, but deframing is performed inone clock domain.

The described sync logic sync hunts by searching approximately half ofthe subframes of each frame. Such a design enables sharing of memory tomaintain state machines for multiple subchannels. Sharing memory reducesthe cost and complexity to implement the deframers. Furthermore, lessspace is used for memory to maintain state machines for deframing.

The techniques shown in the figures can be implemented using code anddata stored and executed on computers. Such computers store andcommunicate (internally and with other computers over a network) codeand data using machine-readable media, such as magnetic disks; opticaldisks; random access memory; read only memory; flash memory devices;electrical, optical, acoustical or other form of propagated signals(e.g., carrier waves, infrared signals, digital signals, etc.); etc. Ofcourse, one or more parts of the invention may be implemented using anycombination of software, firmware, and/or hardware.

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention is notlimited to the embodiments described. Alternative embodiments canimplement the loop controls of the sync hunt logics and deframing logicsin a variety of ways. In addition, as previously described, deframersrunning at a fast clock speed can process bit streams transmitted at aslower clock rate. The increased deframer density leads to alternativeembodiments with the per-alignment state machines for the DS2 and DS3deframers stored in external memory. In another embodiment, a singleexternal memory unit stores the per-alignment state machines for alldeframers. In another embodiment of the invention, each deframing sliceof a network element processes a single DS3 input within a single clockdomain. In another embodiment of the invention, each deframing slice ofa network element processes a single DS3 input within a single clockdomain and shares a single memory unit to store states. In anotherembodiment of the invention, data formats may vary across channels orsubchannels. For example, a deframing slice may receive a DS3 input andan E3 in put as long as the domain clock outruns the sum of the incomingsignal rates. In another exemplary embodiment of the invention, a DS2signal may include three E1 signals instead of four DS1 signals. The E1and DS1 signals can be deframed in the same clock domain.

The method and apparatus of the invention can be practiced withmodification and alteration within the spirit and scope of the appendedclaims. The description is thus to be regarded as illustrative insteadof limiting on the invention.

1. An apparatus comprising: a memory unit to store a set ofper-alignment state machines; a memory controller coupled to the memoryunit, the memory controller to access the set of per-alignment statemachines; a first deframing slice coupled to the memory controller, thefirst deframing slice having a first set of buffers coupled to thememory controller, the first set of buffers to store a first set ofstates from a first subset of the set of per-alignment state machines, afirst set of logic coupled to the first set of buffers, the first set oflogic to sync hunt a first signal with the first set of states and toupdate the first set of states, a second set of buffers coupled to thefirst set of logic, the second set of buffers to store the updated firstset of states, the updated first set of states to be written to thefirst subset of the set of per-alignment state machines; and a seconddeframing slice coupled to the memory controller, the second deframingslice having a third set of buffers coupled to the memory controller,the third set of buffers to store a second set of states from a secondsubset of the set of per-alignment state machines, a second set of logiccoupled to the third set of buffers, the second set of logic to synchunt a second signal with the second set of states and to update thesecond set of states, a fourth set of buffers coupled to the second setof logic, the fourth set of buffers to store the updated second set ofstates, the updated second set of states to be written to the secondsubset of the set of per-alignment state machines.
 2. The apparatus ofclaim 1 wherein the first and second signal have different signalformatting.
 3. The apparatus of claim 1 wherein the first deframingslice further comprises a third set of logic for a second signal format,the first set of logic being for a first signal format.
 4. A networkdevice comprising: a first deframing slice having a first high bit ratesignal format synchronization hunting logic and a first low bit ratesignal format synchronization hunting logic; a second deframing slicehaving a second high bit rate signal format synchronization huntinglogic and a second low bit rate signal format synchronization huntinglogic; a memory to host a plurality of state machines; a memorycontroller coupled with the first and second deframing slices and thememory, the memory controller to perform read and write operationsbetween the low bit rate signal format synchronization hunting logicsand the memory; and a set of one or more machine readable media coupledwith the first and second deframing slices, the set of machine readablemedia having stored therein a set of instructions to cause the first lowbit signal format synchronization hunting logic and the second low bitrate signal format synchronization hunting logic to simultaneouslysynchronization hunt low bit rate signals extracted from different highbit rate signals using the state machines stored in the memory.
 5. Thenetwork device of claim 4 wherein the low bit rate signal format isDS-1.
 6. The network device of claim 4 wherein the state machines areper-alignment state machines.